The present invention generally relates to a magnetic head and more particularly, to a thin film magnetic head (referred to as a thin film MR head hereinafter) that is provided with a magnetic reluctance effect element (referred to as an MR element hereinafter) which is adapted to detect variation of a magnetic field signal that is applied in a direction to the axis that is difficult to magnetize, of a strong magnetic material i.e. ferromagnetic thin film having a uniaxial magnetic anisotropy, so as the axis becomes easier to magnetize, thereby effecting the detection of recorded signals on a magnetic recording medium.
Conventionally, the thin film MR head has been known to have many advantages as compared with a wound type magnetic head. Such a thin film MR head is arranged so that the magnetizing direction in the MR element is altered upon the receipt of a magnetic field signal that is written in a magnetic recording medium such as a magnetic tape or the like, and the variation of an internal resistance of the MR element, in accordance with the variation in the above magnetizing direction, is provided as an external output. Therefore, the thin film MR head is a magnetic flux responding type head, and the thin film MR is capable of reproducing the magnetic field signal, without depending on the transfer speed of the magnetic recording medium. Since the thin film MR head may be readily formed into a highly integrated structure or a multi-element structure, it is possible that the thin film MR head can replace the reproduction magnetic head of a fixed head type PCM recording apparatus intended for a high density recording.
Originally, owing to the fact that the MR element has an induction characteristic showing a square variation with respect to the external magnetic field, in the case where the MR element is constituted as a reproduction head, it is necessary to form the MR element into a stripe shape, and also provide the element with a structure that is applied with a predetermined biasing magnetic field in order to achieve a linear response characteristic. To impress the biasing magnetic field as referred to above, the practice of inducing the biasing magnetic field by causing DC current to flow through a conductor has been known, and another practice of impressing the biasing magnetic field by having a high resistance against magnetic force through the employment of a thin film such as Co-P layer, etc. (Reference may be made to Japanese Patent Application Tokugansho No. 55-126255 and the excerpt from 4th scientific lectures of the Japan Applied Magnetics Society (1980) 5PA-4 entitled "Multi-track Thin Film MR Head") is also known. For the actual application, in the thin film MR head, the MR element is formed on the conductor or thin film with a high resistance against magnetic force as referred to above, through an insulating layer.
Meanwhile, it has been known that a thin film magnetic head normally called a yoke type MR head (referred to as a YMR head hereinafter) can be provided with a magnetic flux introducing path (referred to as a yoke hereinafter) for leading magnetic fluxes that are produced in the magnetic recording medium towards the MR element spaced from the forward end of the head (FIG. 6). A yoke type MR head is more effective for improvements in resolving the signal power and the durability of the MR elements, than an MR head constituted by a single MR element. (Reference may be made to the excerpt from 8th scientific lectures of the Japan Applied Magnetics Society (1984) 14PB-11 entitled "Reproduction Characteristics of Yoke Type MR Heads").
In FIG. 6, a side sectional view of a conventional YMR head is shown (yoke type thin film magnetic head) when taken in a direction perpendicular to a track width direction of a magnetic recording medium.
In FIG. 6, an upper yoke Yu is provided that is normally prepared by a film of permalloy (Ni-Fe alloy) having a thickness in the range of about 0.5 to 1.0 .mu.m so as to serve as a magnetic path for leading the magnetic field generated in a magnetic recording medium R to an MR element H. The MR element H is formed by a deposition film of permalloy (Ni-Fe alloy referred to above) and set for a film thickness in the range of about 200 to 500 .ANG., with the track width in the range of about 50 to 200 .mu.m in a multi-track construction. Moreover, for the application of a biasing magnetic field, a conductor C made of a film of Al and Cu or an Al-Cu alloy, etc. is provided for the application of a biasing magnetic field. Since an actually used recording wavelength actually used is in the order of about 0.5 .mu.m, a head gap G is set in the range of approximately 0.2 to 0.3 .mu.m. The conductor C, MR element H and upper yoke YU as described above are formed respectively through insulating layers L1, L2 and L3 as illustrated. Also formed on a non-magnetic substrate S, for example of crystallized glass, is a lower yoke YL made of a high permeability magnetic film such as a sendust (Fe-Al-Si alloy) film or permalloy by an electron beam deposition process or sputtering, etc.
Since the lower yoke YL made of the high permeability magnetic film requires a film thickness of approximately several .mu.m from the functioning viewpoint of the YMR head, it is needless to say that a thermal expansion coefficient of the substrate S should be in agreement with that of the high permeability magnetic film in order to obtain favorable characteristics of the above magnetic film.
For achieving still better magnetic characteristics, it is necessary that the substrate S has a small surface roughness. However, the surface roughness of crystallized glass generally available is in the range of about 50 to 200 .ANG., and if the high permeability magnetic film is formed on such crystallized glass, the orientation of crystals for the magnetic film will tend to vary, thus making it impossible to obtain the desired magnetic characteristics. Meanwhile, the surface roughness of the high permeability magnetic film formed on the crystallized glass substrate is similar to that of the substrate S, and falls in the range of about 50 to 200 .ANG. in reflection of the surface roughness of said substrate. As a result, at the head gap portion G, the processing accuracy for the gap length is lowered by the undulation on the surface of the lower yoke YL, while characteristic variation in the magnetic film forming the upper yoke YU are undesirably brought about. Furthermore, since a back yoke portion YB is coupled with the lower yoke YL in a state where non-uniformity in crystallinity, etc. takes place, magnetic behaviors becomes uncontinuous, thus making it impossible to obtain favorable magnetic characteristics. Accordingly, in the YMR head as described so far, the influence of noises peculiar to the ferromagnetic member and the influence of strain in the signal waveforms, over S/N ratios of reproduction waveforms are brought as serious problems.